Decorative Steel Composition Having a Textured Cyrstalline Surface

ABSTRACT

A decorative steel article having a textured crystalline surface includes a steel alloy, single crystal or large, macroscopic grained crystal body. The steel alloy includes iron and at least one element selected from the group of nickel and chromium. The steel article has a surface characterized by a decorative, macroscopic, martensitic phase surface feature. The decorative steel article is suitable for sundry applications where an enhanced visual appearance is desirable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The United States Government has rights in this invention pursuant to contract no. DE-AC05-00OR22725 between the United States Department of Energy and UT-Battelle, LLC.

CROSS-REFERENCE TO RELATED APPLICATIONS

None

NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT

None

BACKGROUND OF THE INVENTION

Fe—Ni—Cr steel alloys provide the basis for many widely used commercial and industrial products. There are sundry applications of steels where the alloy is visible and a decorative effect may be desirable. An example of a decorative steel is the well-known Damascus steel, believed to have been developed around 1100 AD. Such alloys are generally used in multi-crystalline form.

There are other forms of decorative metal surfaces, such as those applied by chemical and/or mechanical means. One example is a swirl finish applied by surface abrasion.

BRIEF SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a steel article that includes a steel alloy, single crystal or large-grained crystal body, the steel alloy including iron and at least one element selected from the group of nickel and chromium, the steel alloy body having a surface characterized by a macroscopic, martensitic phase surface feature.

In accordance with another aspect of the present invention, a method of making a steel article includes the steps of: growing a steel alloy, a single crystal or large-grained crystal body, the steel alloy including iron and at least one element selected from the group of nickel and chromium; cooling the steel alloy body to a temperature of no more than 250 K; and returning the cooled steel alloy body to ambient temperature to alter the surface so that the surface is characterized by at least one macroscopic, martensitic phase surface feature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a metallographic image showing a decorative surface feature in accordance with an example of the present invention on a (100) crystallographic surface of an oriented single crystal of a steel alloy comprising 70 wt % Fe, 15 wt % Ni, and 15 wt % Cr.

FIG. 2 is a metallographic image showing a decorative surface feature in accordance with an example of the present invention on a (111) crystallographic surface of an oriented single crystal of a steel alloy comprising 70 wt % Fe, 15 wt % Ni, and 15 wt % Cr.

FIG. 3 is a metallographic image showing a decorative surface feature in accordance with an example of the present invention on a (110) crystallographic surface of an oriented single crystal of a steel alloy comprising 70 wt % Fe, 15 wt % Ni, and 15 wt % Cr.

FIG. 4 is an unmagnified photograph of a steel alloy sample coupon having a decorative surface feature in accordance with an example of the present invention.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a new decorative steel. The new steel composition is formed by initially growing a single, face-centered-cubic (fcc) crystal of a steel alloy comprising iron and at least one of nickel and chromium. The degree of purity of the constituents does not appear to be critical to the invention, but the skilled artisan will recognize that large amounts of some impurities may interfere with the formation of a single crystal or large-grained crystal (also known as macroscopic grained crystal and/or near-single-crystal) that is necessary to carry out the present invention.

It was discovered that the alloys described above in the appropriate compositional range of the Fe, Ni, Cr components undergo a martensitic phase transition when cooled to a temperature less than 250 K.

The martensitic phase transition is highly hysteretic, and when the alloy is returned to room temperature, some residual amount of the martensitic phase is preserved. This remaining martensitic phase forms a distinct, macroscopic decorative feature (pattern) on the surface of the alloy that reflects the symmetry of the single-crystal alloy material. In general, the feature and its contrast are enhanced by the fact that the surface topography of the alloy is altered by the presence of the residual martensitic phase, as well as generally throughout the entire body of the crystal.

This invention relates to the process of forming a decorative steel by growing a single crystal or large-grained crystal of a ternary or binary alloy of Fe—Ni—Cr, Fe—Ni, Fe—Cr, or any more complex alloy that may contain one or more additional constituents as long as said components do not alter the alloy properties in a manner that would prevent the alloy from undergoing a martensitic phase transition on cooling. The single crystal growth may be carried out by a directional solidification process like that employed in the Bridgman/Stockbarger crystal growth method or by crystal pulling (i.e., the Czochralski method) or by any other crystal growth technique capable of producing a single or near-single alloy crystal.

Following crystal growth, the crystal can be shaped by a metal-removal method and not by any forging process in order to preserve the structure and integrity of the single crystal. Any bolt holes, slots, channels, grooves, cutting edges, and the like can be machined into the crystal at this point in the process.

The crystal or large-grained crystal can be polished to a bright finish or satin finish after shaping. Polishing to a bright finish is particularly beneficial for any surface where the desired surface feature is to be visible. A polished surface enhances the visual appearance of the decorative surface feature. The more polished the surface, the more enhanced the decorative feature will be. Conversely, the rougher the surface, the less visible the decorative feature will be.

Following shaping and polishing, the crystal or large-grained crystal is quenched (cooled) one or more times to a temperature at or below the martensitic phase transition temperature less than 250 K. Quenching can be accomplished most easily by immersion in a cryogenic liquid such as liquid nitrogen, which cools materials to 77 K. The rate of cooling is not critical to the invention. Multiple quenching and warming cycles can be used to enhance the decorative surface features.

The crystal is warmed or allowed to warm to ambient temperature. The rate of warming is not critical to the invention. In accordance with the present invention, a pattern of surface features will appear on the surfaces of the crystal or large-grained crystal. Cooling and warming steps can be repeated several times to enhance the decorative surface features.

The skilled artisan will recognize that further polishing of the surface after the surface feature is formed may not be desirable because polishing removes material, and could result in obliteration or diminution of the desired decorative surface features. Blade edges in cutlery applications can generally be sharpened either before or after the quenching process.

The type of decorative feature that is formed can be controlled. Various decorative features having different symmetry patterns can be produced by using surfaces of single crystal alloys with different crystallographic orientations, for example, (100), (110), (111). The orientation of the alloy single crystal can be controlled by “seeding” methods during the crystal growth process or by using x-ray methods to orient the as-grown single crystal that is then cut to produce the desired surface orientation.

EXAMPLE I

Single crystals of a 70Fe—15Ni—15Cr alloy were grown by means of the Czochralski or “crystal pulling” technique. An oriented single-crystal seed was utilized, and the melt from which the crystal was “pulled” was contained in an alumina crucible. Radio frequency induction heating was employed, and the growth process was carried out using a conventional, multi-purpose crystal growth unit. A growth ambient comprising argon at a pressure slightly in excess of one atmosphere was maintained in the growth chamber during the crystal-pulling and ensuing cool down operations.

The alloy samples were prepared by first using Laue back-reflection X-ray methods to orient the crystals. The crystals were then cut using an electric arc discharge machine to expose crystal surfaces that were perpendicular to the (100) direction of the fcc base alloy material. These crystallographically oriented surfaces were lapped and given a final polish using a colloidal silica suspension. The oriented crystals were then transformed by cooling to about 77 K by immersion in liquid nitrogen and subsequently returned to room temperature. The high relief, macroscopic pattern of surface features formed by the intersection of the retained austenite lathes with the {100} austenitic surfaces was subjected to metallurgical and X-ray examination. The clearly visible surface features are shown in the metallographic image of FIG. 1.

EXAMPLE II

The invention was carried out as described in Example I above except that crystals were cut to expose crystal surfaces that were perpendicular to the (110) direction of the fcc base alloy material. The high relief, macroscopic surface pattern formed by the intersection of the retained austenite lathes with the {110} austenitic surfaces were clearly visible, and are shown in the metallographic image of FIG. 2.

EXAMPLE III

The invention was carried out as described in Example I above except that crystals were cut to expose crystal surfaces that were perpendicular to the (111) direction of the fcc base alloy material. The high relief, macroscopic surface pattern formed by the intersection of the retained austenite lathes with the {111} austenitic surfaces were clearly visible, and are shown in the metallographic image of FIG. 3.

FIGS. 1-3 clearly illustrate the ability to alter and control the decorative features formed on the alloy crystal by controlling the orientation of the single crystal material. FIG. 4 shows an example of the macroscopic features of an example of the present invention.

The decorative features will be present on all surfaces that have been cooled in accordance with the present invention. If one desires a pattern having a specific symmetry, then the crystal will have to be cut along a crystalline plane that will yield the desired pattern. The crystal can be cut along various planes with respect to the crystalline planes to produce various, predictable, symmetrical patterns of decorative features. Curved cuts will yield a variety of symmetries consistent with the crystalline structure along the surfaces. It is understood that random orientations of the alloy single crystals may also be employed and that patterns reflecting the various crystal orientations may be visible on highly non-planar surfaces of the material.

The present invention is suitable for sundry applications where an enhanced visual appearance is desirable. Some examples include all forms of cutlery, including custom knives, kitchen and carving knives, pocket knives, hunting knives, military knives, ceremonial knives and the like. Other examples include exposed surfaces of mechanical devices such as handles, knobs, buttons, levers, shafts, cogs, pulleys, faceplates, cover plates, bases, framework, support components, cutters, gauges, hand tools, power tools, automotive components, and the like. Further examples include art objects, crafts, furniture, jewelry, accoutrements, decorative items, and other types of ornamentation.

While there has been shown and described what are at present considered to be examples of the invention, it will be obvious to those skilled in the art that various changes and modifications can be prepared therein without departing from the scope of the inventions defined by the appended claims. 

1-3. (canceled)
 4. A method of making a steel article comprising the steps of: a. growing a steel alloy body selected from the group consisting of a single crystal and a macroscopic grained crystal, said steel alloy comprising iron and at least one element selected from the group consisting of nickel and chromium, said steel alloy body having a surface; b. cooling said steel alloy body to a temperature less than 250 K, c. returning said cooled steel alloy body to ambient temperature to alter said surface so that said surface is characterized by at least one decorative, macroscopic, martensitic phase surface feature.
 5. A method of making a steel article in accordance with claim 4 wherein said steel alloy body comprises a single crystal.
 6. A method of making a steel article in accordance with claim 4 further comprising an additional step, after said growing step, comprising: polishing said steel alloy body.
 7. A method of making a steel article in accordance with claim 4 further comprising an additional step, after said growing step, comprising: shaping said steel alloy body by a metal-removal method.
 8. A method of making a steel article in accordance with claim 7 wherein said shaping step comprises cutting said steel alloy body along a plane respective to a selected crystalline plane.
 9. (canceled)
 10. A method of making a steel article in accordance with claim 4 wherein said temperature is about 77 K.
 11. A method of making a steel article in accordance with claim 4 wherein said cooling step and said returning step are repeated to enhance said surface feature. 